This invention relates to a reactor comprising two coils and a core, and to a step-up circuit comprising the reactor.
There is a need for an interleaved step-up circuit utilizing a reactor because the interleaved step-up circuit can handle large current. An interleaved step-up circuit of this type, which utilizes a reactor, is disclosed, for example, in Patent Document 1 (JPA H10-127049). A reactor, which is utilized in an interleaved step-up circuit of this type, is disclosed, for example, in Patent Document 2 (JPA 2017-168587). Referring to FIG. 10, a reactor 800 of Patent Document 2 has two coils 810, a core 850 and a middle cover portion 880. The core 850 is a cast core which is formed by mixing soft magnetic alloy powder and resin followed by pouring the mixture in a predetermined mold. Each of the two coils 810 is embedded into the core 850. The middle cover portion 880 is made of resin and has an annular flat plate. The middle cover portion 880 is held between the two coils 810.
An electromagnetic property of the reactor 800 of Patent Document 2 is increased as a coupling coefficient of the two coils 810 is increased. In a case where a reactor similar to the reactor 800 of Patent Document 2 is utilized in an interleaved step-up circuit providing two output phases, it is known that the interleaved step-up circuit having a configuration, in which a step-up ratio (duty ratio) is 0.5 while a coupling coefficient of two coils is 1, is most preferred from a point of view of reducing ripple current. Additionally, when the duty ratio is set to a value far from 0.5 in this case, it is also known that ripple current is dramatically increased as the coupling coefficient thereof is increased.
On the other hand, there is a need for a step-up circuit having an available range of a step-up ratio which is suitable for actual use. As understood from above, in order that, in some range of a step-up ratio, a reactor has excellent magnetic properties while a step-up circuit with the reactor has reduced ripple current, a coupling coefficient of two coils of the reactor is required to be appropriately adjusted.